flame retardants

Materials Letters 61 (2007) 2575 – 2578 www.elsevier.com/locate/matlet

Comparison of flammability behavior of polyethylene/Brazilian clay nanocomposites and polyethylene/flame retardants Renata Barbosa a , Edcleide M. Araújo a,⁎, Tomas Jeferson A. Melo a , Edson N. Ito b a

Department of Materials Engineering, Federal University of Campina Grande, Campina Grande/PB, Av. Aprígio Veloso, 882, Bodocongó, CEP. 58109-970, Brazil b Department of Materials Engineering, Federal University of São Carlos, São Carlos/SP, Brazil Received 6 April 2006; accepted 28 September 2006 Available online 16 October 2006

Abstract Polyethylene (PE)/Brazilian clay nanocomposites and PE/commercial flame retardant systems were produced via direct melt intercalation. A montmorillonite sample from the Brazilian state of Paraíba was organically modified with esthearildimethylammonium chloride (Praepagen) quaternary ammonium salt and has been tested to be used in polymer nanocomposites. The dispersion analysis and the interlayer distance of the clay particles were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The flammability behavior of the obtained systems was investigated by horizontal burning tests for HB classification, Underwriters Laboratories (UL94). It was observed that the burning rate of PE/Brazilian clay nanocomposites was significantly reduced in relation to pure PE and PE/flame retardant systems, indicating that the PE/Brazilian clay system was more efficient. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocomposite; Brazilian clay; X-ray technique; Flammability properties

1. Introduction Due to the great interest in modern materials of engineering, much attention has been given to polymer-layered silicate nanocomposites as a result of the potentially superior properties of these materials compared to conventional composites. Minimal addition levels (b 10 wt.%) of organoclays enhance many properties such as mechanical, thermal stability, dimensional and gas barrier, flame retardancy and ionic conductivity properties significantly [1–5]. This new class of materials, according to Komarneni [6], is defined as nanocomposites. To obtain compatible clays with polymer matrices, quaternary ammonium salts with at least 12 carbons atoms have been used in aqueous dis-

⁎ Corresponding author. Tel./fax: +55 83 3310 1178. E-mail addresses: [email protected] (R. Barbosa), [email protected] (E.M. Araújo), [email protected] (T.J.A. Melo), [email protected] (E.N. Ito). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.09.055

persions of sodium smectite clays. In these dispersions, the clay particles or layers must be separated from one another and not be stacked, in order to facilitate the introduction of the organic compounds. As a result, the clay exchange cations are replaced by the organic cations of the quaternary ammonium salt that were adsorbed on the negative sites of the clay surfaces. So, the obtained clay known as organophilic or organoclay is not anymore soluble in water and it will be compatible with polymer matrices, if the organic quaternary ammonium ions were properly chosen [7–10]. The clay layers display a high barrier action and the large thermal stability is related to the lowering of the diffusion of oxygen molecules into the nanocomposites, due to the barrier property of the clay. So, at a lower level of oxygen, which is the main factor for the deterioration of the polymer, the nanocomposite is stronger toward the oxidative decomposition [11–14]. In this work, Brazilian clay and two types of commercial flame retardants were used in PE matrix to evaluate the flammability behavior of PE/clay nanocomposites and PE/ flame retardants systems.

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2. Experimental 2.1. Materials A high density polyethylene (PE, JV-060U) was supplied by Braskem/Brazil and used as a composite matrix. Namontmorillonite (MMT, Brasgel PA, Boa Vista/PB, Northeast of Brazil) supplied by Bentonit União do Nordeste with a cation exchange capacity (CEC) of 90 meq/100 g and an interlayer spacing d001 = 12.5 Å was used as a nanofiller. The quaternary ammonium salt used for the modification of MMT was esthearildimethylammonium chloride — Praepagen (P) with industrial grade supplied by Clariant/Brazil and it was used as received. The commercial flame retardants used were: antimonium trioxide (Sb2O3) as a white solid, supplied by Chemtra Chemical/Brazil, abbreviated as AT, and chlorinated paraffin as powder solid, abbreviated as Chlorez 700 (CL). 2.2. Preparation of organoclays The Na-MMT clay was modified organically with quaternary ammonium salt according to the procedure described by Araújo et al. [9,16,17] and Barbosa [15]. The product obtained with Praepagen was named as P-OMMT.

Fig. 2. TEM photomicrographs of PE systems with (a) 1 wt.% and (b) 3 wt.% of organoclay.

2.3. Preparation of composites PE/P-OMMT composites and PE/flame retardants systems, containing 1 and 3 wt.% of clay or flame retardants, were melt compounded in a counter-rotating twin-screw extruder (Torque Rheometer Haake) operating at 170–200 °C and 60 rpm. Flammability UL94 HB samples were injection-moulded in a Fluidmec machine at 200 °C. 2.4. Characterization of dispersibility of the clay in polymer matrix

Fig. 1. XRD patterns of montmorillonite clay modified with the Praepagen salt (P-OMMT) and for the PE systems with 1 and 3 wt.% of organoclay.

The structure of PE/organoclay composites was characterized by XRD and TEM. XRD measurement was performed using a XRD-6000 Shimadzu diffractometer (40 kV, 30 mA) with 2θ scan range of 2–30° at room temperature, at a scanning

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Fig. 3. Burning rate for PE and PE/commercial flame retardant systems with 1 and 3 wt.%.

Fig. 5. Burning rate for PE, PE/commercial flame retardants systems and PE/POMMT with 1 wt.%.

speed of 2°/min with Cu (λ = 0.154 nm). TEM was carried out on a Philips CM120 with 120 kV. Samples were cryogenically microtomed into ultrathin sections (25–50 nm thick) with a diamond knife using a RMC MT-7000 under cryogenic conditions (− 80 °C) inside the microtoming chamber.

2.5. Flammability test Flammability properties were measured using horizontal burning tests for HB classification according to UL94 [14]. The dimension of the standard bar samples is 125 × 13 × 3 mm. The flammability data reported here are the averages of five samples. 3. Results and discussion 3.1. Structure of PE/clay nanocomposites Fig. 1 presents X-ray diffraction patterns for the P-OMMT clay and for the PE systems with 1 and 3 wt.% of organoclay. The organoclay (P-OMMT) presents two peaks, with interlayer spacings of 29.2 Å and 18.5 Å, in which the intercalation of the salt between the layers of organoclay occurred. Another peak (12.5 Å) is probably due to an incomplete ion exchange; with some residual MMT remaining in the material. For the X-ray diffraction pattern of the nanocomposite of PE with 3 wt.% of P-OMMT organoclay, the main diffraction peak points out to an interlayer spacing of 36.12 Å, due to the intercalation of the polymer chains between the layers of the organoclay. A second broad peak can be attributed to a small part of montmorillonite layers that were not intercalated by PE molecules. A third broad diffraction peak

Fig. 4. Burning characteristics at the beginning of the test for: (a) PE matrix and (b) PE/flame retardant.

Fig. 6. Burning rate for PE, PE/commercial flame retardants systems and PE/POMMT nanocomposite with 3 wt.%.

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(12.5 Å) is probably due to an incomplete ion exchange, with the presence of some residual MMT based in the literature [18]. A similar behavior is shown for the PE with 1 wt.% of organoclay with two peaks around of 20 and 40 Å. This indicates that even increasing the content of organoclay (1 to 3 wt.%) it is still possible to obtain partially intercalated nanocomposites. Fig. 2(a) and (b) shows the TEM images of the PE systems. There exist intercalated clay layers but it can be seen too that some aggregated clay layers are still present in the PE matrix. Therefore, the obtained PE/P-OMMT composites are partially intercalated nanocomposites according to the XRD pattern (Fig. 1). Similar results were presented by Wang et al. [19]. 3.2. Horizontal flammability tests, UL94 HB Horizontal flammability test standardized as UL94 HB [14] was used to investigate the flammability properties of the systems. From Fig. 3 analysis, it shows that the systems with 3 wt.% of additive present a reduction on the burning rate in relation to PE matrix, specifically the PE/CL (3 wt.%) system. In this study, probably the used flame retardants volatilized during their decomposition, diluting the volatile combustibles of the flame and suggesting the formation of a protective oxide char of the product surface, reducing the oxygen diffusion for the reactive centre. These chars retard the heat transfer, and result in the improvement of flammability properties [13,15,20,21]. Fig. 4(a) and (b) presents the burning characteristics at the beginning of the test for PE matrix and PE/flame retardant systems, respectively. It can be observed that the PE/flame retardant systems showed a lower rate of combustion in comparison with the pure polymer and also a lower tendency of dripping. In Fig. 5, it can be observed that the burning rate of the systems with flame retardants (1 wt.%) is reduced dramatically in relation to the PE/ P-OMMT (1 wt.%) nanocomposite. However, it is close to pure PE. With the increase of the amount of clay to 3 wt.% (see Fig. 6), the burning rate value is significantly reduced as compared to PE matrix and the systems with flame retardants. The results demonstrate that the flammability resistance of PE/clay nanocomposite was improved. This is most probably due to the organoclay contributing to reduce the flammability of the systems, suggesting the formation of a barrier property that acts to retard the heat and mass transfer during the combustion and moreover, that the Brazilian clay organically modified is an efficient alternative for the use in nanocomposites. As reported by Valera et al. [22], the thermal degradation of the aliphatic chains in the PP and EP/EVA matrix can be retarded by an improvement in the dispersion and exfoliation of the silicate layers. And moreover, the nanoclays are flame retardants and burning characteristics should be considered to be associated with two possibilities, the gas barrier properties of nanolayers which impede gas diffusion, and the retarding combustion nature of the silicate layers as reported too by Preston et al. [24]. Tang et al. [23] also reported the good gas barrier properties of nanocomposites based on EVA that were assigned to an ablative reassembly of the reticular layers of the silicate in the nanocomposites during thermal-oxidation.

4. Conclusions Polyethylene/Brazilian clay nanocomposites and polyethylene/commercial flame retardant systems were produced via direct melt intercalation. The obtained PE/organoclay nanocomposites were partially intercalated. As expected, the flammability resistance of PE/Brazilian clay nanocomposites

was improved due to the barrier effect of the organoclay during the combustion and the nanocomposite is more effective than conventional PE/flame retardants systems. By adding only 3 wt. % montmorillonite, the burning rate of the nanocomposites was reduced by 17%. This also indicates that the Brazilian clay can be used as a nanoparticle in PE nanocomposites. Acknowledgments The authors express their thanks to PhD Elias Hage JúniorDEMa/UFSCar/Brazil, to PhD Helio de Lucena Lira and to PhD Marcelo Silveira Rabello-DEMa/UFCG/Brazil, to Braskem, to Bentonit União Nordeste, to Clariant, to RENAMI (Rede de Nanotecnologia Molecular e de Interfaces), to FAPESQ/MCT/CNPq (Fundação de Amparo à Pesquisa do Estado da Paraíba), to CNPq and CAPES (Brazilian Research Council) for the financial support. References [1] A. Tidjani, C.A. Wilkie, Polym. Degrad. Stab. 74 (2001) 33. [2] S. Wang, Y. Hu, Q. Zhongkai, Z. Wang, Z. Chen, W. Fan, Mater. Lett. 57 (2003) 2675. [3] M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1. [4] J.W. Gilman, Appl. Clay Sci. 15 (1999) 31. [5] F. Chavarria, D.R. Paul, Polymers 45 (2004) 8501. [6] S. Komarneni, J. Mater. Chem. 2 (1992) 1219. [7] S.A. Body, M.M. Mortland, C.T. Chiou, Am. J. 54 (1988) 652. [8] C.L.V. José, C.A. Pinto, F.R.V. Díaz, P.M. Buchler, In: Anais do 46° Congresso Brasileiro de Cerâmica (São Paulo-SP, 2002). [9] E.M. Araújo, T.J.A. Melo, L.N.L. Santana, R. Barbosa, H.S. Ferreira, A.D. Oliveira, H.L. Araújo, M.M. A'vila Jr., In: Anais do 48° Congresso Brasileiro de Cerâmica (Curitiba-PR, 2004). [10] C. Zilg, P. Reichert, F. Dietsche, T. Engelardt, R. Mülhaupt, Plást. Ind. 64 (2000) (Fevereiro). [11] S.S. Ray, M. Okamoto, Polym. Sci. 28 (2003) 1539. [12] J.B. Gallo, J.A.M. Agnelli, Polímeros: Ciência e Tecnologia, vol. 1, 1998, p. 23. [13] C. Zhao, H. Qin, F. Gong, M. Feng, S. Zhang, M. Yang, Polym. Degrad. Stab. 87 (2005) 183. [14] UL-94: Test for Flammability of Plastic Materials for Parts in Devices and Appliances, Underwriters Laboratories Inc. (UL) (2001). [15] R. Barbosa, M.Sc. Thesis, Federal University of Campina Grande, Campina Grande, Brazil (2005) p. 101. [16] E.M. Araújo, T.J.A. Melo, L.N.L. Santana, G.A. Neves, H.C. Ferreira, H.L. Lira, L.H. Carvalho, M.M. A'vila Jr., M.K.G. Pontes, I.S. Araújo, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 112 (2004) 175. [17] E.M. Araújo, T.J.A. Melo, A.D. Oliveira, H.L.D. Araujo, K.D. Araujo, R. Barbosa, Polímeros: Ciência e Tecnologia, vol. 16, 2006, p. 38. [18] M. Zanetti, L. Costa, Polymer 45 (2004) 4367. [19] K.H. Wang, M.H. Choi, C.M. Koo, Y.S. Choi, I.J. Chung, Polymer 42 (2001) 9819. [20] R.C. Trombini, Ph.D. Thesis, Federal University of São Carlos, São Carlos, Brazil (2004) p. 150. [21] H. Lu, Y. Hu, J. Xiao, Q. Kong, Z. Chen, W. Fan, Mater. Lett. 59 (2005) 648. [22] M. Valera-Zaragoza, E. Ramirez-Vargas, F.J. Medellin-Rodríguez, B.M. Huerta-Martínez, Polym. Degrad. Stab. 91 (2006) 1319–1325. [23] Y. Tang, Y. Hu, S.F. Wang, Z. Gui, Z. Chen, W.C. Fan, Polym. Degrad. Stab. 78 (2002) 555. [24] C.M.L. Preston, G. Amarasinghe, J.L. Hopewell, R.A. Shanks, Z. Mathys, Polym. Degrad. Stab. 84 (2004) 533.